libtorc-video: Use AV_PKT_FLAG_KEY as well as key_frame to track corrupt
frames
libtorc-qml: Add initial video playback support.
libtorc-core: Add TorcLocalContext::RegisterQThread and
TorcLocalContext::DeregisterQThread

- these allow QThreads (as created and owned by Qt in this case) to
access the database(s) and setup logging.
libtorc-qml: Register the Qt QML render thread internally.
libtorc-core: Simplify logging thread registration.
libtorc-qml: Fix semi-random video whiteouts
libtorc-qml: Only mark the node dirty when video frame is updated.
libtorc-qml: Actually clear the default video framebuffer